Having gone over the basics for the cross grip shoulder throw/seionage, an essential component for the throw is the pull. It's also called "Kuzushi" in Judo, meaning unbalancing or something vaguely to that effect.
There's 3 parts to an effective throw (massive generalization, but bear with me) which I emphasize: advantageous grip, throwing with movement, and making space for my entry.
The failures in throwing virtually all stem from the absence of one or more of the above. Either you're throwing from an inferior grip or neutral grip and thus your opponent easily counters.
You attempt to throw from a static position and your opponent is heavy in base and easily defends.
You attempt to throw and you haven't opened up the space necessary for your entry and attempt to pull him across you or from behind or never slide into optimal throwing position.